Why wouldn't they charge for the work? Other than they are nice people who would prefer to support a customer and lose money...? Our shop's usual policy is that we will swap parts on a warranty replacement frame free if the bike was purchased through our shop, but we will charge for a frame-up bike build if the bike was purchased elsewhere. Shops are not reimbursed by factories to do this work, so they either eat the cost of something they are not responsible for in order to maintain good customer relations, or they rightfully charge for the work...

20% off crash replacement usually means that much off a whole new bike, not just a frame.